1. Field of the Invention
The present invention relates to a lithium secondary battery with a high energy density and, more particularly, to a lithium secondary battery with metal lithium coated on the electrode surface of a negative electrode or a positive electrode or both, in which the negative electrode includes a material which can be alloyed with lithium and having a capacity per unit weight of 700 mAh/g to 4300 mAh/g, the positive electrode is made of a transition metal oxide capable of reversibly intercalating and deintercalating lithium, and the entire reversible lithium storage capacity is included in the positive electrode (i.e., when the battery is fabricated, the entire reversible lithium storage capacity is included in the positive electrode and greater than a lithium capacity dischargeable in an available voltage range), and lithium of a metal form does not remain in the negative electrode and the positive electrode after an initial activation charge.
2. Discussion of the Related Art
In order to spread environment-friendly vehicles, the development of electric power for the vehicles is being accelerated. Important requirements for the spread of the electric vehicles include the price per energy, energy per weight, safety, and durability. Today, development centers on a lithium secondary battery having excellent durability and a long life span, but there is an epoch-making improvement for the price per energy and energy per weight. To this end, a focus is concentrated on a lithium alloy material including a large amount of lithium per unit weight and elements capable of forming an alloy, such as aluminum (Al), tin (Sn), and silicon (Si), in order to form a negative electrode of a high capacity. However, the material is problematic in that the loss of an initial irreversible capacity is great.
Meanwhile, in order to improve the energy density of the lithium secondary battery, a technique for increasing the capacity of a positive electrode material is being developed. In particular, an Li(Ni, Co, Al)O2 material or an Li(Ni, Co, Mn)O2 material with stabilized LiNiO2 has an advantage in that they can greatly increase the capacity of the lithium secondary battery because they have a high reversible capacity per unit weight. However, the material is problematic in that it has a great reaction with an electrolyte in the full charge state because it needs to be charged with a high voltage in order to achieve a high capacity characteristic. Accordingly, the material has a serious aging characteristic in the life span according to a long-term storage or cycle. Further, a battery using the material is insufficient in the safety of a unit cell when the capacity of the unit cell is increased and thus problematic in that thermal runaway and ignition are generated when being overcharged or stored in a hot box of 150° C.
In order to overcome the above problems and achieve a higher capacity, PCT 2006/112674 provides a lithium secondary battery using an active material, not including lithium, for a positive electrode and a lithium transition metal oxide with a high irreversible lithium capacity. The lithium secondary battery of this patent can be made with a higher capacity because a transition metal oxide, not including lithium, for the positive electrode with a high capacity can be used. Further, the lithium secondary battery is excellent in the safety and long-term durability because voltage when the battery is fully charged is not high. The lithium secondary battery, however, has a drawback in that most of the transition metal oxide (i.e., an inactive solid compound) remains within the battery after lithium is used, thereby deteriorating the energy density, because the lithium important to increase the capacity is stored in an initial lithium transition metal oxide.
As a scheme for providing sufficient lithium and also significantly increasing the energy density per weight, research becomes active on a battery using metal lithium. For example, in order to improve a dendrite problem according to charge and discharge, PCT/JP2004/007877 proposes a negative electrode for a lithium metal secondary battery in which metal lithium is deposited on an insulating base and an inorganic solid electrolyte film is formed on the metal lithium. Meanwhile, U.S. Pat. No. 7,247,408 proposes a lithium metal electrode coated with multiple layers of a single one conductive layer and a polymer layer in order to inhibit the formation of lithium dendrite. Further, in order to further increase the surface processing effect and also effectively form a thin metal lithium film, Korean Patent No. 10-0496306 proposes a method of forming a metaplasia film on a base, forming a current collector on the metaplasia film, and lithium metal is deposited on the current collector, thereby being capable of reducing a spatial loss due to the base film, suppressing deformation resulting from heat, and obtaining a deposition lithium layer with a high degree of purity. However, the above methods can steadily increase reversibility according to lithium charge and discharge, but is problematic in that it does not fundamentally prevent ignition and explosion resulting from metal lithium when the battery is used in abnormal environments or the battery is overheated upon misuse.
Meanwhile, a technique for previously doping metal lithium and fully consuming the doped metal lithium through activation has recently been applied to a lithium ion capacitor. In this case, the initially inserted lithium is compressed into the surface of an electrode in the form of a lithium metal plate or a lithium metal electrode is inserted into a third electrode and then electrochemically doped on a lithium storage electrode. For example, Japanese Unexamined Patent Application Publication No. Hei8-107048 provides a method of sealing the carbon electrode of an electric dual layer capacitor with a metal foil physically brought into contact with the surface of the carbon electrode and chemically doping lithium on the carbon electrode while raising temperature. However, this patent is problematic in that lithium metal difficult to mechanically handle must be used in order to dope only a necessary amount of lithium (i.e., very thin within 5 to 20 micron) on a common capacitor or a lithium battery. Further, this patent has a difficulty in that if this patent is applied to a capacitor or battery with a high capacity and a wide electrode area, the thin metal lithium foil must be adhered to the wide area. As yet another example, WO98/33227 provides a technique for introducing practically applicable metal lithium into a third electrode and doping lithium on a carbon negative electrode by electrochemically dissociating lithium from the third electrode. However, in order to uniformly dope lithium ions on the carbon negative electrode having a wide area, a current collector having holes through which the lithium ions can freely penetrate in forming negative electrode and positive electrode plates must be used. If the current collector having holes is used, there are problems in that the mechanical strength of the electrode is weakened and a process of manufacturing the electrode is complicated as compared with a case in which a uniform metallic foil current collector with no hole is used.